1. Field of the Invention
The present invention relates to testing devices for use in measuring the accuracy of nuclear imaging instruments, especially instruments used for diagnostic purposes in nuclear medicine.
2. Description of the Prior Art
A number of different nuclear imaging instruments have been devised for diagnosing patient ailments and conditions. The field of use of such instruments is known as the field of nuclear medicine. Nuclear imaging instruments are advantageous in that they can produce images of circulatory conditions within soft tissue organs of a patient's body without exploratory surgery.
In the practice of nuclear medicine, a low level dosage of a tracer radioisotope, such as technitium-99 m is injected into a patient. The tracer radioisotope is carried in the patient's blood stream to the patient's internal organs, such as the heart, liver, kidneys, etc. The tracer radioisotope emits gamma rays, a portion of which pass from the patient's body and are detectable by nuclear imaging instruments.
There are several types of nuclear imaging instruments. A relatively simple nuclear imaging instrument is known as a scanner. A scanner is equipped with a gamma ray detector, such as a sodium iodide NaI(Tl) scintillation crystal. The gamma ray detector responds to an incident gamma ray by emitting a flash of light. A photodetector, such as a photomultiplier tube, is positioned to continuously view the sodium iodide crystal, and to emit a voltage pulse as gamma rays are detected. The scanner is moved in a raster pattern outside of a patient's body adjacent to the patient's organ of interest. A record of the voltage pulses from the photomultiplier tube is kept, thereby creating a "map" or image of the location at which gamma radiation was detected. An unusually high concentration of radiation indicates an internal lesion in the organ scanned, while an unusual absence of radiation indicates a circulatory blockage.
A further development beyond the scanner was the scintillation camera. The scintillation camera includes a large scintillation crystal proximate to which an array of photodetectors are positioned. Rather than moving from one point to the next, as does the scanner, the scintillation camera is able to view an entire field of view and is able to determine the location of incident gamma rays in the crystal by ascertaining the intensity with which the resulting flash of light is received by the various photodetectors in the array.
Scintillation cameras have also been used in a scanning mode to scan large areas of the body of a patient. Such scanning is typically carried out with a translating converging collimator. Scintillation cameras have also been used with slant-hole collimators. In such a utilization the scintillation camera is rotated in an orbit about an axis essentially perpendicular to the surface of the patient's body and focused on a plane beneath the surface of the patient's body. The use of scintillation cameras in either of the foregoing ways is referred as emission tomography imaging.
A further development in scintillation detector imaging is the technique known as Single Photon Emission Computed Tomography (SPECT). According to this technique a scintillation camera is aligned on a radial axis essentially perpendicular to an imaginary line passing through the patient from head to foot. The camera is then rotated in orbit about this imaginary line and nuclear events are detected and processed as the scintillation camera is moved in this fashion. With each revolution of the camera about the imaginary line passing through the patient from head to foot, a set of contiguous images of transverse cross section of the patient's body is produced. Different sets of transverse sections or "slices" are obtained by positioning the camera longitudinally relative to the imaginary line and driving the camera in a revolution about the patient's body. A description of a simple Single Photon Emission Computer Tomography technique appears in my prior U.S. Pat. No. 4,057,726.
In order to calibrate and check the accuracy of nuclear imaging instruments, test structures, known in the field as phantoms, are utilized. Prior phantom designs have involved devices in which radioactive or "hot" sources are interspersed at known spacing intervals within surrounding non-radioactive or "cold" regions. One typical prior art phantom is a lucite plate phantom. In this test structure a plurality of flat, parallel, lucite plates are arranged in a container at spaced intervals from each other. The lucite plates are divided into several groups. The plates within each group are of equal thickness and are spaced from each other a uniform distance. The thickness of the plates and the spacing therebetween is different in each group. The interstitial volumes between the lucite plates contain a radioactive source, such as technetium 99 m.
Other prior nuclear imaging phantoms have been produced in forms which encase radioactive sources within a structure, the shape and gamma ray attenuation properties of which simulate a human organ or body. Such conventional test phantoms may employ concentrated radioactive sources which are imaged as "hot" spots or small non-radioactive structures within a homogeneous radioactive source which exhibit "cold" spots in the image produced.
While conventional prior art phantoms have been adequate for purposes of testing, calibrating and adjusting nuclear imaging devices which are stationary, which are scanned, or which are employed in emission tomography imaging, the test phantoms heretofore available have been unacceptable for these purposes in Single Photon Emission Computerized Tomography. Because the technique of SPECT imaging produces a much higher quality of image as contrasted with prior nuclear imaging devices and techniques, the test phantoms heretofore available produce images in which software errors, malfunctioning components and misadjustments are simply not ascertainable. Nevertheless, such conditions must be ascertained if SPECT imaging is to be utilized to its full capabilities as a tool of nuclear medicine.